1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser and a laser array thereof, and more particularly to a surface emitting semiconductor laser of polarization control type used in optical information processing and optical communications, or as a light source of an image forming apparatus using light, and a laser array in which plural surface emitting semiconductor lasers are arrayed.
2. Description of the Related Art
In laser application technology, which often requires that an emitting spot be circular, since a surface emitting semiconductor laser has the property that an emitting spot can be easily made circular, and can be two-dimensionally integrated, it is in the limelight, centering on the field of communications as a light source having a high degree of usability. By the way, except for surface emitting semiconductor lasers of the so-called distortion type having a distortion quantum well active layer, since surface emitting lasers do not have the anisotropy of oscillation threshold gains within a plane perpendicular to an emission direction, polarized light faces to all directions with an equal probability. That is, they have a different polarization direction for each laser device.
If polarization directions differ depending on laser devices, where optical devices having polarized wave dependence such as mirrors and polarization beam splitters not provided with special coatings are used in combination, a change in optical properties occurs in light beams having passed through the optical devices. Use of a large number of laser devices by use of an identical optical system would cause variations in light quantities and other problems because of the influence of polarization properties different for each laser device. For this reason, efforts have been so far made to stabilize the polarization of laser devices to one direction.
Although various methods for polarization control has been heretofore devised, they cannot be said to be satisfactory in terms of perfect control, and polarization control methods are presently contemplated in various quarters.
Recently, a polarization control method using a tilted substrate has become dominant.
There is described in xe2x80x9cIEEE Photonics Technology Lettersxe2x80x9d, Tenth Volume, page 633, a surface emitting laser of selective oxidation type of an indium gallium arsenic (InGaAs) system that uses a tilted substrate with gallium arsenic (GaAs) (311) B face as anormal direction. An optical wave (active) region formed by selective oxidation is of rectangular shape having a diameter of 6 xcexcmxc3x973 xcexcm, with a major axis in [xe2x88x92233] direction and a polarization direction also in the [xe2x88x92233] direction. By using such an extremely special tilted substrate, the surface emitting laser of selective oxidation type provides the properties of polarization mode suppression ratio 30 dB or more.
Although this method seems to be highly effective for polarization control, since the GaAs tilted substrate with the (311) B face as a normal direction is designed for special uses, it is more expensive than general-purpose substrates with (100) face as a normal direction. Since crystal growth is different in condition from growth on the (100)face, which is widely performed, huge costs and a great deal of time will be required to find growth conditions excellent in reproducibility for actual use.
There is described in xe2x80x9cApplied Physics Lettersxe2x80x9d, 71st Volume, page 741, a surface emitting laser of proton injection type of an aluminum gallium arsenic (AlGaAs) system fabricated using a (001) substrate (different in notation from the above (100) substrate and crystallographically denotes an equivalent crystal face orientation) tilted by approximately 2xc2x0 toward (111) A direction. The surface emitting laser of proton injection type has the property of a polarization mode suppression ratio exceeding 400 at the maximum. The substrate tilted by approximately 2xc2x0 used herein is easily available because of widespread use as a low dislocation (highly smooth) substrate and has almost the same crystal growth condition as when a substrate not tilted is used. Therefore, this method can be said as a simple method for polarization control.
However, an obtained laser is a surface emitting laser of proton injection type.
Since a laser of proton injection type is a device employing heat caused by current injection called thermal lens effects, it has the following property problems attributed to its construction: (1) a current must keep being injected for a certain period of time to form a waveguide by heat, which is required to cause laser oscillation; and (2) the light response property is bad in addition to a threshold current as high as several milliamperes. For this reason, recently, a surface emitting laser of selective oxidation type is going mainstream which narrows a current and confines light by forming an oxidation region and thereby provides high output with a low threshold current. However, in the surface emitting laser of selective oxidation type, since it is known that the shape of an oxidation region affects the polarization property, and the polarization property is highly governed by a fabrication process history, the polarization direction of the surface emitting laser of selective oxidation type cannot be sufficiently controlled in the same way for a surface emitting laser of proton injection type.
The surface emitting laser of proton injection type described in the above literature provide a polarization mode suppression ratio of 100 or more before a light output value reaches a peak, and is required to provide higher output and be capable of controlling polarization.
Although a laser array driven with plural laser devices arranged is particularly required to be controlled so that the polarization directions of the devices are identical, a measured polarization mode suppression ratio is merely for one device, and it is not guaranteed that, when the devices are formed into an array, all of the devices would exhibit the same property without variations in polarization directions.
Therefore, the present invention has been made in view of the above circumstances and provides a surface emitting semiconductor laser that, with a relatively simple construction, can control the polarization of laser beam to a given direction and obtain a low threshold current and high output. Also, the present invention provides a laser array that has no variations in polarization properties among plural laser devices arranged on a single substrate.
According to an aspect of the present invention, the surface emitting semiconductor laser has upper and lower reflecting films formed on a main face of a semiconductor substrate to sandwich an active layer, at least one of the upper and lower reflecting films including a selective oxidation layer oxidized in a circumferential part thereof, wherein the main face of the semiconductor substrate is tilted with respect to a face containing a reference crystal axis, and the selective oxidation layer is formed by oxidizing a layer to be oxidized from a circumferential part thereof, wherein the circumferential shape of the layer, when cut by a face parallel to the main face, has at least no singular point and is macroscopically smooth.
According to the present invention, by using a semiconductor substrate tilted with respect to a face containing a reference crystal axis, oxidizing a layer whose circumferential shape has substantially no singular point and is macroscopically smooth, from a circumferential part thereof, and forming a selective oxidation layer selectively oxidized in a circumferential part thereof, a non-oxidation region having a circumferential shape almost analogous to the circumferential shape of the oxidation layer is formed, the anisotropy of oscillation threshold gains of optical mode is induced, and polarization is controlled to a given direction without causing switching or the like.
Since the passage of little current through the oxidation region causes a refractive index waveguide to be formed within a face parallel to the main face of the semiconductor substrate to narrow a current and confine light, devices having excellent properties such as stable horizontal mode and low threshold current are obtained.
According to another aspect of the present invention, in the surface emitting semiconductor laser, the face containing the reference crystal axis has a crystal face orientation crystallographically equivalent to the (100) face.
According to another aspect of the present invention, in the surface emitting semiconductor laser of the present invention, the main face is tilted at an angle in the range from xe2x88x925xc2x0 to +5xc2x0 to [110] direction with respect to [100] direction.
According to another aspect of the present invention, in the surface emitting semiconductor laser of the present invention, when the layer to be oxidized is cut by a face parallel to the main face, a circumferential shape thereof is circular or elliptic.
According to another aspect of the present invention, in the surface emitting semiconductor laser of the present invention, when the layer to be oxidized is cut by a face parallel to the main face, the circumferential shape is elliptic, and the direction of the major axis is [011] or [01-1] direction.
According to another aspect of the present invention, in the surface emitting semiconductor laser of the present invention, the non-oxidation region of the selective oxidation layer has a diameter within a range in which fundamental transverse mode oscillation is obtained.
According to the above-described aspects of the present invention, since each of the aspects provides an optimum condition for deriving the anisotropy of oscillation threshold gains of optical mode, synergistic effects of these aspects provide further increased polarization control for the surface emitting semiconductor laser.
According to another aspect of the present invention, in the surface emitting semiconductor laser of the present invention, an upper wiring is disposed at an upper portion of an emission part in a direction extended at an angle in the range from at least xe2x88x9210xc2x0 to +10xc2x0 with respect to the projection of the reference crystal axis to the semiconductor substrate main face.
According to the present invention, since stress is applied to an active region along a wiring direction because of the difference of thermal expansion coefficients between the wiring and materials disposed at lower portions of the wiring, by making an off angle direction of the substrate and the extension direction of the wiring almost identical, anisotropy attributed to the stress dependence of oscillation threshold gains occurs and the surface emitting semiconductor laser with polarization controlled to a given direction can be obtained.
According to another aspect of the present invention, the laser array has the plural surface emitting semiconductor lasers of the present invention arranged on a single substrate, i.e. monolithic.
According to another aspect of the present invention, equally in the laser devices, the anisotropy of oscillation threshold gains of optical mode is induced and polarization is controlled to a given direction. Therefore, the laser array without substantial variations in polarization properties among the laser devices can be obtained.
According to another aspect of the present invention, in the laser array of the present invention, an upper wiring is disposed at an upper portion of an emission part in a direction extended at an angle in the range from at least xe2x88x9210xc2x0 to +10xc2x0 with respect to the projection of the reference crystal axis to the semiconductor substrate main face.
According to the present invention, since stress uniform in directionality among plural devices is applied to an active layer, the surface emitting semiconductor laser array without sybstantial variations among the devices and with identical polarization directions can be obtained.